s390/blacklist: Perform subchannel scan only when needed

Move scheduling of a subchannel scan to those instances where new
devices may actually have become available. This reduces unnecessary
scan work in case devices were added to the blacklist.

Signed-off-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Reviewed-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>